Shear-band spacing controlled by Bridgman solidification in Dendrite/BMG composites

Abstract

The paper reports that the shear-band spacing can be controlled by the Bridgman solidification technique for a model alloy of Zr 38.3 Ti 32.9 Nb 7.3 Cu 6.2 Be 15.3 . The volume percent of the glass phase is almost independent of the withdrawal velocity. The shear-band spacing reaches a minimum of about 3 μm with withdrawal velocity of 0.8 mm/s. The optimized mechanical properties, such as the fracture strength of ~2100 MPa and plastic strain of ~19%, have been obtained. The large plastic deformation is due to the dislocation slips in the dendrite phase and shear-band propagations in the glass phase. Shear-band spacing, Bridgman solidification, BMG compositesBulk metallic glasses (BMGs) exhibit many unique properties, such as ultrahigh strength which is about two or three times stronger than the crystalline materials, high elastic strain limits as high as about 2%; super-plastic behavior in the supercooled liquid region; ductile for the smaller sample; and low melting temperature because of the near eutectic composition. Thus future research directions are mainly focused on the BMG composites, and high-entropy alloys [1][2][3][4][5][6] .BMG composites are usually prepared by the copper-mould suction casting. However, the samples prepared by copper-mould suction casting contain inhomogeneous dendrites in the glass matrix, due to the decreasing cooling rates from the outer surface to the center of the rod-shape samples, which leads to large deviation from that of the reported mechanical properties [7] . Another drawback of the suction casting comes from the pores that easily form during the suction of the viscous liquid into the copper mould and that also deteriorate the tensile mechanical properties. Figure 1(a) shows the sample used in the tensile tests, Figure 1(b) shows a pore on the fracture surface after the tensile test, and Figure 1(c) shows the tensile stress-strain curve. The inset in Figure 1(c) is the XRD pattern which shows the alloy Cu 46 Zr 47 Al 7 is of BMG composite [8] . The formation of pores reduces the actual area for the tensile loading, and leads to the lower fracture strength.This paper used Bridgman Solidification to overcome the problem by adjusting the withdrawal velocities (V) in a controlled manner, and composites with different length scale of dendrite were obtained. In this paper, the relationship between the shear-bands spacing (λ) and the technical parameter, V, was founded, and this provides a technical method to improve the plasticity of the dendrite/BMG composites.

Proposition of a Growth Law as a Function of Solidification Parameters for Monotectic Alloy Systems

“…There is no agreement as to the validity of these growth laws to characterize the growth of monotectic systems. The vast majority of these studies are based on the use of solidification devices of the Bridgman type that impose a stationary regime of heat extraction [35], with few evaluating solidification processes of monotectic alloys under transient conditions, considering the effects of the flow of particles rich in solute in the remaining liquid volume, and correlating the measured thermal solidification parameters with the monotectic structures got, which brings the experiences closer to the industrial reality [28,36]. There are also few studies that analyze the influence of the growth direction on the interphase spacings and on the dimensions of bismuth, lead, and indium particles [37].…”